Intervertebrates react selectively to the presence of foreign agents in their tissues. Molluscs and other invertebrates frequently respond to invading parasites by encapsulating the parasites with aggregations of blood cells called hemocytes. In other cases, however, the host organism seems to lack any cellular defense or the cellular reaction is less marked or delayed. The proposed work will be devoted to the study of molluscan hemocytes (a) during snail-parasite interactions, and (b) after experimental insertion of foreign particles into snails so as to reveal alterations of carbohydrates and polyanions of the surface membranes of the parasites and hemocytes when the snail recognizes or fails to recognize, the foreign nature of the parasite or abiotic particle. The work will focus on intramolluscan forms of schistosomes, the human flatworm parasites. Moreover, the work will go beyond the problem of the cellular basis of internal defense mechanisms of host-parasite phenomenon in a variety of biological situtations such as parasitism, morphogenesis, fertilization, and many pathological conditions. Cell surface carbohydrates, which show a great diversity in structure, may provide cells with a variety of specific and efficient recognition surfaces in cell adhesion. Electron microscopy in conjunction with lectin and carbohydrate cytochemistry, will be used to study the role of hemocytes in snail-parasite interactions.